(1) Field of the Invention
The present invention relates to mechanical touch input systems such as touch pads and touch screens used in fixed or mobile devices, such as point of sales terminals, kiosks, laptops, monitors, PDAs, cell phones, UMPCs and more, that require the touch component to be fully constrained in two directions (x & y), but requiring unencumbered freedom to translate in the third direction (z-direction).
(2) Description of Prior Art
The concept of using multiple force sensing sensors to register, measure and triangulate the touched position of a touch screen has been a known concept for more then twenty years, however, to produce a high quality touch screen solution has proven difficult.
Over the last few years the performance of force sensors has improved substantially and the component cost has been reduced to commercially viable prices. In addition, there has been software advances, creating an environment ready for high volume product implementations of touch screens based on force sensing. There is however still one major problem that must be overcome, the mechanical implementation.
For the typical force based touch screen implementation, the input device consists of a free standing touch screen lens or the actual LCD module. This touch screen lens rests on 3 or more force sensing sensors that are placed on a rear cover or some type of rigid surface, such as a PC Board or a back plane. The overall mechanical housing holds the different components in place as a system with different types of mounting mechanisms, which allow for movement in the lens, yet keeps the lens in place and pressed towards the force sensors. The force sensors are electrically connected, sometimes signal amplified, and converted from analog to digital so that sensor readings may be provided to the touch screen control software running on the device processor or on a separate micro controller. It should also be mentioned that most force sensors are designed to measure vertical forces and have minimal to no tolerances to measure forces accurately that are not applied exactly straight into the sensors measurement plane.
The mounting mechanism of the touch screen is an active part of the touch screen and refers to the way the top lens, which is used as an input device, and the sensors that measure the position and the amount of exerted force, are mounted on base plate. Until now, production of force based touch screen devices with high accuracy has been a challenge due to large errors in reading and interpreting the force. The errors occur because the forces introduced by the mechanical devices are not countered sufficiently and very difficult to account for. For example, the lens needs to allow for movement in the z direction, but must be fixed in the xy-plane in order to not introduce side movement and to minimize side forces. The lens also needs to be pre-loaded to ensure that the touch screen lens always remains in contact with the force sensors, even if the unit is moving or being turned upside-down. Adding a pre-loading solution will also introduce non-linear forces that are difficult to correctly account for, especially since the direction of gravity is typically unknown.
For example, U.S. Pat. No. 4,511,760 to Garwin et al. issued Apr. 16, 1985 shows a force sensing data input device responding to the release of pressure force. The input surface is provided with a transparent faceplate mounted on force-sensing piezoelectric transducers. Preferably, four piezoelectric transducers are provided, one at each corner of a rectangular opening formed in the frame. To determine the point of application of force on the input surface, the outputs of the four transducers are first summed. To constitute a valid data entry attempt, the sum must exceed a first threshold while the user is pushing on the input surface. When the user releases his finger, a peak of the sum is detected, which is of opposite polarity from the polarity of the sum for the pushing direction. The individual outputs of the four sensors at the time that the peak of the sum occurs are used to calculate the point of application of the force. This mechanical construct is using spring clips to pre-load the sensors and to keep the mechanical assembly in place. These spring clips could theoretically minimize movement in xy-plane, but will provide non-linear additions to the total forces as the lens is touched since the spring loading force will change as the lens is pressed (and moved) in the z-plane. The position of the pre-loading springs will also add to the complexity since they are adding forces that bends the lens over the sensors.
U.S. Pat. No. 5,038,142 to Flower et al. (IBM) issued Aug. 6, 1991 shows a touch sensing display screen supported by stiff springs having essentially uniaxial freedom of motion. Strain gauge sensors are applied directly to the springs and a processor calculates the X, Y and Z coordinate information. This configuration is similar to the above-referenced Gawin patent inasmuch as it employs a basic spring construction for allowing movement in z-direction but restricting the xy-plane. Again there are likely a lot of uncontrollable forces. U.S. Pat. No. 6,879,318 by Chan et al. issued Apr. 12, 2005 shows a touch screen mounting assembly for a liquid crystal display panel LCD including a bottom frame, a backlight panel seated in the frame and that has a plurality of pressure-sensitive transducers mounted thereon, a liquid crystal display panel, and a top frame for exerting pressure when mounted to the bottom frame such that a plurality of compressible springs biases the LCD panel towards the bottom frame when touched or contacted by a user. The bottom and top frame assembly with backlight panel are mounted therein on springs, with an overlying LCD panel. Spring loaded mounting screws will allow for movement in the z-plane and pre-loading, but non-linear forces from pre-loading as well as lens friction will be present.
Two core problems have been identified in the foregoing references and other existing mechanical solutions for force based touch screens.
The first problem identified is the extremely small tolerances required for the mechanical build-up. The issue is that the touch screen needs to be extremely rigid, since if the lens bends part of the force will be captured in the lens material or even lost in heat dissipation. Unfortunately, in making the sensor mounting surface as well as the lens (which rests on top of the sensor) extremely rigid, there is no longer any accommodation for mechanical tolerances. The sensor must fit the components exactly (within 1/100s of a mm), or else the lens will ether not be in contact or must be forced down and be bent) through pre-loading. Due to the very small movement allowed within the force sensor and the use of rigid and parallel surfaces, keeping the top plate at the same distance and parallel to the base consistently, before and after the pressure is applied, remains a challenge both from a production as well as a measurement perspective.
The second core problem is the interference from other forces. Typical problems arises from non-linear forces, when there is contact and friction between the lens and other mechanical components, and from pre-loading, where the applied preloading creates non-linear additional forces as the lens is pressed down and some of the pre-loading forces are neutralized through the new and additional forces loading the lens towards the sensors. In addition, there may be bending in the materials, where some of the force is lost into side forces and heat.
These problems surface as the following symptoms:
1) Low accuracy: Approximately 1-10% of total distance between sensor in positioning error
2) Repeating the same operation such as drawing a line, but starting from the opposite direction does not yield the same results (due to interference of lateral forces an/or actual side movement of lens transferred to the force sensors)
3) Greater amount of the touch force is required in order to compensate for the mounting mechanisms' pushback forces
4) Loss of sensitivity since a high minimum force (approximately >20 gf) is required in order to eliminate inaccuracies from non-linear forces from friction, pre-loading or material bending.
5) An elaborate mounting mechanism of the top plate is needed to keep the sensors flat. The mounting adds to the forces that interfere with the measurements.
6) Difference in positioning calculation, that is different size and type of accuracy error, at different force levels.
Current precision instruments that incorporate precision bearing slide mechanisms try to offset the drag or friction when a shaft or feature is actuated through them. However, these parts can be somewhat bulky and pricey. In much more size constrained assemblies, or high volume manufacturing applications, a smaller or lower cost solution is needed.
It would be more advantageous to provide a suspension system for a touch-screen display that does not introduce any additional friction or non-linear forces to the touch screen system. It would also be desirable to reduce the dependency on extremely tight mechanical tolerances, which is a common problem in the above-described prior art force-based touch screens, and it is, therefore, an objective of current invention to address these needs with a more efficient mechanical construction.